Current detecting devices of this type include a current detecting device described in JP 2007-33222A (hereinafter referred to as Document 1), for example. The conventional example described in Document 1 includes a core (closed magnetic circuit core) shaped like a circular ring into which a conductor is inserted, a coil (winding) wound around the core, and an AC power supply configured to apply an AC excitation voltage to the coil. Furthermore, in the conventional example described in Document 1, the current flowing through the coil is converted to a voltage signal by a detection resistor, peak values of the voltage signal are held by positive and negative peak hold circuits, and a voltage resulting from combining the outputs of the two peak hold circuits is added to the AC excitation voltage to thereby be fed back to the coil.
That is to say, when a magnetic flux is generated on the periphery of the conductor by a current flowing in the conductor that passes through the core, a feedback voltage resulting from combining the outputs of the positive and negative peak hold circuits becomes asymmetric in positive and negative polarities. By applying the feedback voltage to the coil a magnetic flux having a direction and strength that cancels the above-mentioned magnetic flux is generated. Therefore, the magnitude of the current flowing in the conductor can be measured by measuring the feedback voltage with the detection resistor.
Incidentally, the conductor, in which the detection target current flows, generally forms a closed circuit. Accordingly, a transformer is to be formed by the conductor that forms the closed circuit and one coil that is wound around one core, and accordingly an induced current is to flow in the conductor. As a result, there is a problem with the conventional example described in Document 1 in that noise (noise terminal voltage) is superimposed in the closed circuit that includes the conductor, when the AC excitation voltage is applied to the coil.